Hatha Yoga Pradipika has a statement which sounds very simple and has been misinterpreted badly (paraphasing):
Whoever can contain his semen, no matter what else he does, will succeed as a yogi.
... and now we have all these people shilling "semen retention" on /fit/ and /x/, counting days of abstinence and constantly "failing". That's not how you do this. Read in context, it's easy to see that this is not how you're supposed to do it. Another practice in the book is to create skin lotion from your semen mixed with the ashes of burnt cow dung. How do you do that if also practice semen retention at any cost?
What I found was that when entering the deepest meditative state, "delta" (compare beta, alpha, theta), essentially the same as being asleep, any intrusive thoughts will have to filter through the lower states of sleep to get there, and they'll be watered down, quite similar to how a neural network rationalizes an algorithm... If remaining unmoved for a longer time in this state, external energies will be powerless and you will naturally subdue them. That also goes for sexual energy building up. Remain here for long enough, deep enough, being firm enough and the impulse creating sexual release will be forced into a position where it's under your control, leading the energy upwards inside your body instead of outwards.
It is of no importance to practice "semen retention" physically in itself. It only matters if you can turn this impulse inwards instead of letting it be in contact with external energies, when you attempt to do it. The "failing" lies in not being able to do this when you want, not in failing to always do this. That's extremist and will only make you discouraged.